In the frigate-bird, the deeply scooped membrane between the toes shows that structure has begun to change.

frigate bird

noun

any bird of the genus Fregata and family Fregatidae, of tropical and subtropical seas, having a long bill with a downturned tip, a wide wingspan, and a forked tail: order Pelecaniformes (pelicans, cormorants, etc)Also called: man-of-war bird